Generally, certain lasers may be sensitive to optical feedback from a laser beam reflected back to the laser from a work surface. Laser back-reflection may cause laser instability or damage to certain types of lasers. For example, fiber lasers are typically very sensitive to back-reflections from a work surface. If such reflections are not properly blocked, e.g., if there is a return path for such reflections to couple back into an output fiber, it is possible that both the output fiber and gain fibers may be damaged. Further, a seed laser may be damaged in a high-power master oscillator fiber amplifier device by reflected light that is amplified on its way back to the seed laser. In many laser processing systems, such as laser micromachining systems, it is therefore desirable to prevent such back-reflections from finding a path back into the output fiber.
One solution for reducing or avoiding back-reflection in laser processing systems is to use a Faraday isolator, such as the isolators manufactured by Electro-Optics Technology, Inc. of Traverse City, Mich. Placing a Faraday isolator in the beam path after the laser source or output fiber blocks the back-reflections in free-space before the back-reflections return to the laser source or output fiber.
For example, FIG. 1A is a block diagram of a typical laser system 101 including a laser source 102, an isolator 104, a beam delivery subsystem 106, and a scan head 108. The laser source 102 outputs a laser beam 103 received by the isolator 104. The isolator 104 allows the laser beam 103 to propagate in only one direction (e.g., as indicated by the arrow) through the beam delivery subsystem 106 and the scan head 108 to a desired location on a work surface 122. Thus, the isolator 104 blocks back-reflections from the work surface 122 through the scan head 108 and beam delivery subsystem 106 so as to prevent the back-reflections from reaching the laser source 102.
For a linearly polarized laser system, the isolator 104 may be a polarization dependent isolator that includes an input polarizer (not shown), a Faraday rotator (not shown), and an output polarizer (not shown). Although shown following the laser source 102, the isolator 104 may be located at other locations along the beam path such as in the scan head 108 or within the beam delivery subsystem 106 (which may include, e.g., galvonometers, steering mirrors, lenses, and other optical elements).
For a randomly polarized laser system, a polarization independent optical isolator may be used to prevent or reduce back-reflections. For example, FIG. 1B is a block diagram of a typical fiber laser processing system 100 that includes an output fiber 110, a collimator assembly 112, a Faraday isolator 114, and a focusing lens 116. The output fiber 110 directs a divergent laser beam 118 from a laser source (not shown) to the collimator assembly 112. The collimator assembly 112 collimates the divergent laser beam 118 to provide a collimated laser beam 120 to the Faraday isolator 114.
The Faraday isolator 114 allows the transmission of light in only one direction. The collimated laser beam 120 transmits through the Faraday isolator 114 to the focusing lens 116, which focuses the beam onto a work surface 122. Because the incident laser beam's path is perpendicular to the work surface 122, a reflected laser beam 123 (shown in dashed lines) travels along the same path as that of the incident laser beam 120, in the reverse direction, through the focusing lens 116 to the Faraday isolator 114. The Faraday isolator 114, however, prevents the reflected laser beam from continuing along the reverse path back to the output fiber 110.
If the laser beam 118 exiting the output fiber 110 is randomly polarized (as it often is), the Faraday isolator 114 is configured to be polarization insensitive. As shown in FIG. 1B, for example, a polarization insensitive Faraday isolator 114 may include an input birefringent wedge 124, a Faraday rotator 126, and an output birefringent wedge 128. While such isolators are commercially available, they are usually quite bulky (especially when configured for use with high-power beams), expensive, and add complexity to system alignment. Optical isolators may also be subject to thermal lensing effects in high power applications.
Another approach to reduce or avoid back-reflections is to “tilt” the entire beam delivery subsystem with respect to the work surface 122 so that the angle of incidence of the beam hitting the work surface is not 90 degrees. For example, FIG. 2 is a block diagram of another typical fiber laser processing system 200 having a tilted beam delivery subsystem (e.g., the output fiber 110, the collimator assembly 112, and the focusing lens 116) such that the path of the incident laser beam 120 is non-perpendicular to the work surface 122.
As a consequence of tilting the beam delivery subsystem, the path of the reflected laser beam 123 from the work surface 122 is angularly separated from the path of the incident laser beam 120. A portion 210 of the reflected laser beam 123 may propagate back through the focusing lens 116 to the output fiber 110. However, the angular separation between the path of the incident laser beam 120 and the reflected laser beam 123 corresponds to a spatial separation which, in turn, substantially prevents the back-reflected beam 123 from coupling into the output fiber 110. The amount of spatial separation between the paths of the incident laser beam 120 and the reflected laser beam 123 is proportional to the focal length of the focusing lens 116 and the angular tilt (with respect to the work surface 122) of the beam delivery subsystem. Thus, the amount of spatial separation may be increased by increasing either the focal length, the angular tilt, or both the focal length and the angular tilt.
As discussed below, while tilting the entire beam delivery subsystem with respect to the work surface 122 reduces back-reflections, it also tilts the focal plane with respect to the work surface 122. This leads to variations in the spot size and fluence at the work surface 122. These variations deteriorate process performance.